openvswitch: properly refcount vport-vxlan module

After 614732ea, no refcount is maintained for the vport-vxlan module.
This allows the userspace to remove such module while vport-vxlan
devices still exist, which leads to later oops.

v1 -> v2:
 - move vport 'owner' initialization in ovs_vport_ops_register()
   and make such function a macro

Fixes: 614732ea

 ("openvswitch: Use regular VXLAN net_device device")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>